The 20 primary schools that received threatening emails on Wednesday morning in Carlow town and county opened on Thursday.High-visibility Garda patrols are to continue for several days in the areas surrounding all 42 primary schools in the county.The threatening email arrived unexpectedly into computer systems at about 7am on Wednesday and, as a result, at least 11 schools decided to close.Details of the mail included a threat of mass shootings along with a photograph of three guns. It is understood the photograph is a stock image.Garda are following a number of lines of inquiry, one being that the email was sent by a group that specifically targets institutions and organisations to get a large-scale policing response.The email, the Garda added, was routed through another country. The credibility of the message is being investigated.Mayor of Carlow Paul Doogue said it was a “very frightening” time for all those impacted by the email threat.“I really hope that whoever did this is caught and the full weight of the law is enforced,” Doogue said. “Carlow schools, to the best of my knowledge, are open today but some parents may not send their children as they are still a little bit worried and upset and I can understand that - it’s a very frightening time for the children, parents and teachers.“I want to compliment the gardaí and all involved who are trying to trace down this person or persons from whatever country they are in.”Supt Anthony Farrell said high-visibility Garda patrols around all schools began on Wednesday and would “continue” following the threat. Lines of communication have been opened at a senior level with the Department of Education and Youth.